No. 587] 



CALIFORNIA DEER-MICE 



ences of pigmentation are capable of being relatively fixed 

 germinally. Bringing these three sets of facts together, 

 they are, in my opinion, most readily harmonized on the 

 assumption that the direct effects of humidity upon the 

 organism may finally become fixed through heredity. 20 

 But I grant that this assumption is as yet far from 

 proved. 



A study of the effects of captivity forms a new line of 

 inquiry which was hardly considered when the present 

 researches were undertaken. For some reasons, how- 

 ever, this line of investigation now seems quite as prom- 

 ising as the search for climatic effects. In the first place, 

 my comparatively meager results in this field already re- 

 veal striking differences between the wild stock and the 

 individuals of the first generation reared in captivity. 

 These differences relate to absolute size (the domesti- 

 cated generation being smaller), to length of tail and foot 

 (both shorter in the domesticated) and to the length of 

 the femur and the pelvis, which differ in the same direc- 

 tion. In the case of the femur, in particular, the differ- 

 ences are striking, even upon the most casual inspection, 

 and they are absolutely certain statistically. They hold 

 for both sexes and for both of the subspecies which have 

 thus far been tested in this regard. Thus far, no certain 

 effects upon the cranial capacity of the captive lots have 

 been detected. 



Such differences as those found probably have nothing 

 to do with the direct effects of external conditions, but 

 are due to the activities of the animal. For this reason, 

 they are of much higher value as a test of the Lamarckian 

 principle, which could help us most in explaining the per- 

 fecting of active parts through use or of their degenera- 

 tion through disuse. 



The experimental results just referred to suggested 

 the possibility that some of the differences found to occur 

 in nature might have had a functional basis. The larger 



